1. Technical Field
The present invention relates to granular abrasive material and, more specifically, to a method of producing it.
2. Prior Art
Fused alpha-aluminum oxide corundum produced in electric smelting processes, for example in electric arc furnaces, is the major material used for the production of abrasive tools by the entire abrasives industry.
The raw materials used to produce the normal fused forms of corundum are bauxite, prepared either directly from the natural state or chemically into calcined alumina, as well as additives, e.g. reduction coke and scrap iron. The calcined alumina is extracted by thermal processing from aluminium oxide primarily obtained by the Bayer process. The calcined alumina contains varying amounts of alpha-aluminium oxide and representatives of gamma-aluminium oxides depending on calcining temperature and time.
Grinding tools, made with the bauxite or alumina forms of corundum obtained from melts, when tested under established test conditions, give a specific time-related degree of abrasion and are found to have a specific service life, which can be measured in terms of the volume or weight of material ground away. Improvements of the grinding performance of normal forms of fused corundum have been achieved by thermal after-treatment processes (blue calcination of bauxite forms of corundum) or by alloying with other metal oxides, e.g. chromium oxide or zirconium oxide. Fed. Rep. German Patent Specification No. 22 27 642, describes fused corundum consisting of aluminium oxide and zirconium oxide in a eutectic composition (approximately 57 weight % Al.sub.2 O.sub.3 and 43 weight % ZrO.sub.2). The material has a two-phase, microcrystalline solidification structure, obtained by the spontaneous cooling of the melt. This material, which will be referred to as "Zirkonkorund", has a superior grinding performance (time-related wear and service life) relative to the normal fused forms of corundum. The high cost of zirconium oxide and the expensive process for the necessary fast cooling, make granular "Zirkonkorund" from five to six times more expensive than normal fused corundum.
The increased performance of "Zirkonkorund" abrasive material as compared with normal corundum falls abruptly when grinding metal materials, such as steel, as the grain size is reduced and substantially levels off at a grain size of P80, that is the grains will pass through an 80 mesh sieve as defined by FEPA (the European Federation of Manufacturers of abrasive products). This result is also observed with other high performance abrasives.
A highly effective granular, abrasive corundum based on fused aluminum oxide is described in Fed. Rep. German Patent Application No. 32 19 607. This high grade granular fused corundum abrasive material is made by dispersing very fine crystalline aluminum oxide monohydrate in nitric acid and water containing other dissolved metal-containing sintering aids to produce a gel which, after careful drying, is prereduced down to abrasive grain size. The material is then calcined at a temperature between 250.degree. C. and 800.degree. C. to remove the chemically bound water and the acidic residues, primarily nitrogen oxides, which are extremely toxic and damaging to the environment.
The granular material is then heated to sintering temperatures of up to 1650.degree. C. until a density of at least 85% of the theoretical density is achieved.
Similar methods of production of granular abrasive, corundum materal are described in European Patent Application No. 0 024 099A and U.S. Pat. No. 4,518,397. These methods require that the dispersed aluminum oxide monohydrate used as the raw material can only be contaminated with a maximum of 0.05% by weight of alkali or alkaline earth metal ions.
In the method described in European Patent Application No. 0 152 768A the colloidal solution or gel is additionally ground in a vibration grinding mill, as a result of which a sintered product of increased density and with no large surface areas on the particles is produced. The material is formed of homogeneously orientated alpha-aluminium oxide crystallites. It is common to all the four methods mentioned above that they can be carried out only by a colloidal solution/gel process with a very finely dispersed aluminium oxide monohydrate of boehmite type. These relatively expensive raw materials can only be obtained by means of the hydrolysis of aluminium organic compounds. This together with the costly process technology results in such colloidal solution/gel abrasives costing several times more than normal sintered forms of corundum.
Although sintered corundum material, e.g. tabular alumina, costs less it has a definitely worse abrasion performance relative to fused forms of corundum and is therefore quite unsuitable for general use in grinding tools.